Engineers are constantly seeking ways to improve fuel injector performance in order to accomplish various goals, such as reducing undesirable engine exhaust emissions. One strategy that has been adopted in this regard is the use of a hydraulic direct control needle valve to open and close the nozzle outlets of the fuel injector. In such fuel injectors, a needle control valve is moveable between positions that either expose a closing hydraulic surface on a needle valve member to high pressure or low pressure. While this innovation has greatly improved the ability to electronically control fuel injection characteristics, there remains room for improvement.
One area in need of potential improvement relates to the response time of the direct control needle valve to an electrically actuated needle control valve. Among other things, the response time can be improved if the volume of the needle control chamber, which applies either high or low pressure to the closing hydraulic surface of the needle valve, can be reduced. One strategy for accomplishing this goal is to locate the needle control valve and its associated electrical actuator deep inside the fuel injector in close proximity to the direct control needle valve. Another potential strategy for reducing response time is to reduce the travel distance of the needle control valve member, which acts as a pressure switch in exposing the closing hydraulic surface of the direct control needle valve to either high pressure or low pressure. While these two strategies appear to have promise, their implementation can potentially introduce new problems.
In one class of directly controlled fuel injectors, a solenoid is the chosen type of electrical actuator to control movement of the needle control valve. In order for these relatively small fast moving electrically actuated valves to behave predictably, the armature air gap should be known in order to produce predictable results. In order for the valve to perform in a manner consistent with other valves produced in mass production, the air gap should be uniform among valves in order to insure consistent performance in one fuel injector compared to another. These issues are further complicated by the fact that the armature air gap should be relatively small in order to extract the maximum performance from the interaction between the solenoid coil and stator relative to the armature. Furthermore, because the needle control valve wants to be located in close proximity to the direct control needle valve, it might have to be located under a distortion region within the fuel injector, which relates to the area underneath a plunger within a fuel injector. In other words, each time a plunger reciprocates within a fuel injector, fuel is raised to extremely high injection pressure levels. In turn, these pressure forces cause some measurable amount of distortion within the fuel injector. While these distortions are relatively small in magnitude, they can approach a magnitude that is on the same order as an armature air gap tolerance. Thus, in some situations it is possible for component distortion within a fuel injector to cyclically alter the needle control valve's armature air gap to the point that it briefly distorts the armature air gap out of acceptable geometrical tolerances. As such, the predictability of performance is undermined, and the variability in distortion from one fuel injector to another undermines the ability to mass produce valves that behave consistently between different fuel injectors.
Another potential problem introduced by locating an electrically actuated needle control valve in close proximity to the direct control needle valve relates to packaging considerations. In other words, the act of locating the needle control valve deep within the fuel injector further pressures packaging considerations that insure that all of the various fuel injector performance functions and structure can be packaged in an available envelope of space.
One potential strategy for desensitizing injector performance to geometrical distortions taking place within the fuel injector is to employ a two way valve as the needle control valve instead of a three way valve. In the case of a two way valve such as that shown in Heavy Duty Diesel Engines—The Potential of Injection Rate Shaping for Optimizing Emissions and Fuel Consumption”, presented by Messrs. Bernd Mahr, Manfred Dürnholz, Wilhelm Polach, and Hermann Grieshaber, Robert Bosch GmbH, Stuttgart, Germany, at the 21st International Engine Symposium, May 4–5, 2000, Vienna, Austria. The control valve member merely moves into and out of contact with a single seat, rather than moving between two seats as in the case of a three way valve. While such a two way valve strategy can potential assist in desensitizing fuel injector performance to component distortion, it necessarily suffers from other draw backs rendering it less than desirable. For instance, a two way valve strategy inherently results in substantial wastage of high pressure fuel since the fuel injector is controlled by opening its high pressure fuel passage directly to a low pressure drain during injection events. Even when flow restrictions are placed in the control passageways, the amount of fuel spilling leakage can be so substantial as to undermine the overall efficiency of the fuel injection system.
The present invention is directed to one or more of the problems set forth above.